I know how to add Google to the search area in Firefox on Linux Mint. But I don't get any search suggestions, even with them enabled. I add it via the http://www.linuxmint.com/searchengines.php page instructions.

Also, even if the search area changes to Google, the location bar still uses DuckDuckGo. How could I change this?I have nothing against DuckDuckGo or Yahoo!, but the results I get from them aren't as good (IMO) as the ones given by good ol' Google.

To get search suggestions you need to install a search plugin that supports search suggestions. You can install these for Google from Mozilla's Mycroft Project: http://mycroft.mozdev.org/. Make sure to install one that has the A9 icon (meaning it is an OpenSearch plugin, which usually will support search suggestions).